Calcium-yttrium silicate oxyapatite laser materials

ABSTRACT

A COMPOSITION OF MATTER WHICH CAN BE USED AS A LASER CRYSTAL AND WHICH CAN BE DOPED WITH SENSITIZER IONS HAS THE EMPIRICAL CHEMICAL FORMULA CAY4-X(SIO4)3O:AX WHERE A REPRESENTS A LASING ION SELECTED FROM ND AND ER AND X HAS A VALUE FROM .001 TO 1.

Jan. 4, 1972 R. H. HOPKINS ETAL 3,632,523

CALCIUM-YTTRIUM SILICATE OXYAPATITE LASER MATERIALS Filed Sept. 22, 19693 Sheets-Sheet 1 N V R r AI V I u Q L D SENSITIZER GROUND STATEACTIVATOR D FIG. 1

INVENTORS RICHARD H. HOPKINS, GEORGE w. ROLAND, WILLIAM 0. PARTLOW &W'TNESSES KENNETH B. STEINBRUEGGE ATTORNEY EMISSION PER EXCITATIONQUANTUM, ARBITRARY UNITS INTENSITY, ARBITRARY UNITS Jan. 4, 1972 RHOPKlNs ET AL 3,632,523

CALGIUM-YTTRIUM SILICATE OXYAPATITE LASER MATERIALS Filed Sept. 22, 19693 Sheets-Sheet 2 Fluorescence Spectrum CaY Nd (Si0 O l l I 0.9 1.0 1.1

WAVELENGTH, MICRONS FIG. 3

Excitation Spectrum l I I I I I I 0.3 0.4 0.5 0.6 0.7 Q 0.8 0.9

WAVELEIIGTH, MICRONS FIG. 5 I

Jan. 4, 1972 HOPKINS ET Al. 3,632,523

CALCIUM-YTTRIUM SILICATE OXYAPATITE LASER MATERIALS Filed Sept. 22, 19693 Sheets-Sheet 5 EINHHSVH Oi HAIlV'lHH 'NOISSIWSNVHl "IVNOIlQVEH UnitedStates Patent O U.S. Cl. 252-301.! F 7 Claims ABSTRACT OF THE DISCLOSUREA composition of matter which can be used as a laser crystal and whichcan be doped with sensitizer ions has the empirical chemical formulaCaY., (SiO O:A where A represents a lasing ion selected from Nd and Erand x has a value from .001 to 1.

BACKGROUND OF THE INVENTION Energy transfer from one fluorescent specieto another or among fluorescent species of the same kind, is afundamental process in luminescence. Before the advent of lasers, energytransfer was widely utilized in commercial phosphors, such as those usedin fluorescent lamps to improve their efliciency, and was extensivelystudied in connection with organic phosphors.

With the advent of lasers, energy transfer processes have taken onadditional importance as a means for improving the efiiciency ofoptically pumped lasers. The work on fluorescent lamps was concernedmainly with the transfer of energy between transition metal ions ofdifferent types. In contrast, investigations on laser materials havebeen principally concerned with energy transfer from transition metalions to rare-earth ions, or energy transfer from rare-earth torare-earth ions.

The basic aim of lase energy transfer can be described as follows: givenan ion which has desirable spectroscopic properties (i.e., it emits in adesirable frequency region with a suitable bandwidth etc.), but which isonly a weak or ineflicient absorber of the excitation energy, one mustfind another ion, which has desirable absorption properties and whichcan transfer its energy efl'iciently and rapidly to the emitting ion.The emitting ion is called the activator or lasing ion and the absorbingion is called the sensitizer. Energy transfer occurs from the sensitizerto the activator 'It has been demonstrated in U.S. Ser. No. 732,593,filed on May 28, 1968, and assigned to the assignee of this invention,that the mineral fluorapatite, Ca (PO )F, is an excellent host forsensitizer and/ or activator ions. Suitably doped fluorapatite exhibitshigh gain and low threshold characteristics. Large single crystals ofthis doped material are prepared by Czochralski growth fromstoichiometric melts at temperatures of about 1650 C.

Our invention relates to a composition of matter suitable as a lasercrystal in a resonant cavity of a laser generator. Our laser materialsare based on silicate oxyapatite hosts doped with neodymium or erbium.Within the limits of our measurements, these materials melt congruently.They melt at considerably higher temperatures (about 2100 C.) thanfluorapatite and exhibit a higher material strength. Although theexistence and synthesis of some silicate oxyapatite powders generallyhas been disclosed, as for example by Jun Ito in 53 AmericanMineralogist 890; growth, doping and laser application of large singlecrystals of our materials has not been previously considered.

In addition to the crystalline laser materials of this in- 3,532,523Patented Jan. 4, 1972 "ice 'vention, other related crystalline lasermaterials are described in United States patent applications Ser. Nos.859,673; 859,754 and 859,753, all filed on Sept. 22, 1969 and assignedto the assignee of this application.

SUMMARY OF THE INVENTION It is the prime object of this invention toprovide a new and improved high strength composition of matter for useas a laser crystal in the resonant cavity of a laser generator.

This invention accomplishes the foregoing object by providing a silicateoxyapatite laser crystalline material having the empirical formula:

where A represents an activator ion (lasing ion) that is responsible forlaser output. A, the activator ion, is selected from Nd or Er. A, whichion is the lasing ion, in the crystal can be determined by measuring thefrequency of the laser oscillations and from known spectrographic data.Generally only one lasing ion will oscillate at a time. S represents asensitizer ion which need not be present in the crystal. The sensitizerion must be matched to the lasing ion. The value x can vary between.001-1 with a preferred range between .00 l.3 and y can vary between 0to (4-1:) with a preferred range between 0 to 1.0. Y, yttrium, isconsidered a host constituent and the prime constituent for whichactivator and sensitizer ions are substituted.

These materials have low threshold characteristics and low gain allowingimproved energy storage. They also have high material strength. Thesematerials provide a laser crystal capable of withstanding Withoutstructural distortion, significantly higher pumping energies thanfiuQrapatite. Little segregation is observed in doping with neodymiumand erbium. This reduces serious crystal problems which exist in mosthosts due to variation in dopant segregation along the crystal caused bytemperature fluctuations during growth.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thenature and objects of the invention, reference may be made to thefollowing drawings in which:

FIG. 1 shows energy levels of sensitizer and activator ions indicatingtransitions pertinent to energy transfer;

FIG. 2 shows a laser generator utilizing the laser crystal of thisinvention in association with a radiation source in a resonant lasercavity;

FIG. 3 shows the fluorescence spectrum of a polycrystalline sample ofCaY Nd (SiO O. This figure is uncorrected for spectral transmission ofthe spectrometer and detector sensitivity. The uncorrected spectrometercaused the l.06-micron group to appear relatively weaker than it shouldbe;

FIG. 4 shows the absorption spectrum of a single crystal of CaY Nd (SiOO, beam perpendicular to the c axis of the crystal and through athickness of 0.2 inch; and

FIG. 5 shows the excitation spectrum of the infrared fluorescence from apolycrystalline sample of CaY gNd 30 DESCRIPTION OF THE PREFERREDEMBODIMENTS The silicate oxyapatite host material of this invention hasthe formula CaY (SiO O. This host contains ion sites which willaccommodate both rare earth and transition metal ions. The crystalstructure of CaY (SiO O, where Y is a host constituent, is hexagonalwith a unit cell formula of Ca Y (SiO O This host material has anapatite structure (space group P6 /m). The Si+ ions are in SiOtetrahedra. Two sets of ions are present. One set is co-ordinated withSi in the SiO tetrahedra and the other set occurs along the c axis (twoions per unit cell) with each ion co-ordinated by three cations (calciumor yttrium) in the plane of the horizontal mirror in 1 6 m. Two types ofcations sites are present (Ca or Y; and Ca or Y The activator ionsand/or transition metal or rare earth sensitizer ions can substitute forY and Ca in the host material. This will be a substitution of some ofthe five Ca and Y cations in the host CaY (SiO 0.

The host materials of this invention use the rare earth ions Nd+ or Er+as their activator (lasing) ion. The ion concentration of theseactivators can vary in the host from about .020 to 20 atom percent ofthe iive cations (one Ca and four Y cations) in the host CaY (SiO O. Thepreferred range of activator is from about .02 to 6 atom percent. Belowthe preferred range there is generally not enough optical absorption andabove the preferred range there may be concentration quenching. Thus, xhas a preferred value between .001 and .3, i.e., (.001=x)/ 5 cationsequals .020 atom percent and (.3=x)/5 cations equals 6 atom percent.However with improved flash sources for special applications it isuseful to have the value of x greater than .3. The sensitizer ion S hasa preferred range for y from 0 to l i.ev the ion concentration of Spreferably varies from about 0 to 20 atom percent (1=y/5) of the fivecations in the host.

In accordance with this invention sensitizer ions may be used tosensitize the rare earth activator ions Nd+ or Er+ Referring now to FIG.1 which illustrates the Various steps involved in non-radiative energytransfer: (1) The sensitizer ion absorbs a photon of external radiationof energy r, lifting it from the sensitizer ground state D to an excitedstate A. (2) The sensitizer subsequently decays to a lower metastablestate N, by the emission of a photon r or by a non-radiative process.(3) Once lattice relaxation about the sensitizer metastable state hastaken place, the sensitizer is either free to radiate a photon r or totrans fer its energy to an activator ion, as indicated by R. (4) If theelectronic transitions in both the sensitizer and activator are electricdipole transitions, the dipole field of the excited sensitizer canintroduce a dipole transition in a nearby activator, thereby raising theactivator to an excited state A, with a simultaneous return of thesensitizer to its ground state. (5) This transition transfers a quantumof energy from the sensitizer to the activator. Once excited, theactivator can decay to a lower metastable level N, through emission ofphotons, and can eventually decay to its ground state D either directlyor via an intermediate level D". Reference may be made to D. L. Dexter,J. Chem. Phys, vol. 21, 1953, p. 836 for detailed descriptions of theseenergy transfer processes.

The requirements in non-radiative transfer for eflicient transfer ofenergy from sensitizer to activator are: (1) A reasonable overlap inenergy between the sensitizer emission band and an absorption band ofthe activator. (2) High oscillator strengths in both sensitizer andactivator. 3) A relatively high intrinsic radiative quantum efliciencyfor both the sensitizer and activator. In addition to the abovecriteria, there are several other criteria of a more general nature fora useful sensitizer. These are: (1) The sensitizer should absorbradiation in a spectral region where the activator has little or noabsorption, (2) The sensitizer should absorb in a region where the pumplamp radiates appreciable energy, and 3) The sensitizer should notabsorb where the activator emits, or have any adverse effects on theradiative efficiency of the activator. For eificient energy transfer tooccur it is necessary that the rate of transfer (R in FIG. 1) be morerapid than the rate of decay of the sensitizer to its ground state (r"in FIG. 1).

The sensitizing ions that may be used in the composition of thisinvention would include transition metal and rare earth ions, which arecapable of (a) absorbing radiation TABLE 1 Suitable Activator sensitizerHost ion ions +3 +2 CaY4(SlO-l)30 3 In the preparation of the lasercrystalline material of this invention, 19.6863 grams of CaCO- 86.61 87grams of Y O 3.3098 grams of Nd O- and 403853 grams of silicic acid weremixed together. All reactants were of luminescent grade (greater than99.9% purity). The ingredients were then placed in an iridium crucibleand melted at approximately 2090 C. as measured by an uncorrectedoptical pyrometer.

Crystals were pulled from the melt at 2090" C. using the standardCzochralski technique, well known in the art and described in an articleby I. Czochralski in Zeitschrift fur Physikalische Chemie, Vol. 92,pages 219-221 (1918). A recent description of the process is found in anarticle by H. Nassau and L. G. Van Uitert in Journal of Applied Physics,Vol. 31, page 1508 (1960).

The furnace was surrounded by a quartz cylinder attached to theapparatus by means of a neoprene gasket and a brass flange. Insulationfor the iridium crucible was provided by /2 inch thick zirconiaquadrants stacked into a cylinder. Thermal distribution throughout themelt was controlled by adjusting the crucible in the field of the Workcoil and by changing spacing of the zirconia quadrants and the topplate. The power source was a Westinghouse 30 kv.-a. motor-driven 10,000cycle generator with a water cooled copper work coil. The pullingapparatus was designed such that pull rates between 1 and 40 mm./hr. androtation speeds of 10-70 r.p.m. could be used. Temperature wascontrolled by using the output of a sapphire light pipe leading to aradiamatic detector which fed the output into an L and N Azarrecorder-controller. The voltage from the recorder-controller inassociation with an L and N current adjusting type relay supplied theinput current of a Norbatrol linear power controller. The Norbatroloutput voltage supplies the necessary field excitation required by the10,000 cycle generator.

The seed was held on a water cooled shaft which was threaded toaccommodate an iridium chuck. The crucible and chuck were protected fromoxidation by an argon atmosphere. Oriented seeds were used for growth.These were obtained by starting with a polycrystalline seed obtainedfrom a slow-cooled melt. Crystals were grown as large as 4 inch indiameter and 1 inch long. Cooling rates of the pulled crystals variedfrom two to six hours.

The crystalline materials grown and containing Nd or Er lasing ions areuseful as laser crystal rods in simple lasers and in more complicatedlaser applications such as Q switched lasers, both of which aredescribed in detail in chapters 3 and 4 and especially pages 132-160 ofThe Laser by W. V. Smith and P. P. Sorokin, McGraw-Hill, 1966, hereinincorporated by reference.

A simple schematic illustration of a typical laser generator is shown inFIG. 2 of the drawings. Between reflectors 20 and 21 there is a resonantlaser cavity containing the laser crystal 22, a radiation source means23 such as a flash lamp which provides pump energy to the crystal, andpossibly a Q switching means shown by dotted lines. Reflector 20 ispartially reflecting to permit the escape of light beams of coherentradiation 24 whereas reflector 21 is highly reflective.

The basic principle involved in Q switching a laser is to allow a veryhigh population inversion to be built up by making the laser cavitylosses excessive while the laser is being pumped, thereby preventing thelaser from oscillating prematurely. When a strong inversion is attained,the conditions are suddenly made favorable for oscillation by rapidlymaking the cavity losses very small, so that a condition of large netamplification is suddenly realized. The Q switch could, for example,contain a metallo-organic compound in solution such as a phthalocyaninewhich absorbs light from the crystal. The pumping energy input from theflash lamp increases until amplification in the laser crystal overcomesthe loss due to absorption in the Q switch cell and the laser begins toemit coherent light weakly. A very small amount of this light bleachesthe solution which then becomes almost perfectly transparent to thelight. At that instant there is suddenly a gaint pulse of lightcontaining all the stored energy in the laser rod.

One of the crystals pulled at a rate of /1 inch per hour from a melt atabout 2090 C. showed laser action at a 1.06-micron wavelength. Thecrystal composition was CaY Nd (SiO O. This grown boule was ground andpolished. The finishing procedure on the rod end resulted in polishedends parallel to better than 6 are seconds and plane to wavelength of Helight. It was in the form of a 0.205" diameter, 0.938" long circularrod. It was tested in a 2.7" cylindrical reflecting cavity.

The laser head used in all pulse tests was a cylindrical pyrexreflecting cylinder 75 mm. in diameter and 76 mm. long having tworeflecting pyrex end plates with holes machined for the lamp and rod.Front surface evaporated aluminum coatings were used and overcoated with10 quartz for protection. Resonator reflectivities were both 99.2%. Theoutput resonator was optically flat, the other resonator had a one metercurvature. A PEK Xel-3 xenon flashlamp was located diametricallyopposite the laser rod with a center to center spacing between the lampand rod of 0.6 inch. This flashlamp was a broad band emitter with a peakemission around 5800 A. The laser rod was supported in a double-wallpyrex cylinder filled with a water filter solution of NaNO to prevent UVfrom reaching the laser rod being tested. The flashlamp was powered by acharged 340 ,uf. capacitor which was discharged through a 150 h.inductor in series with the lamp. The maximum energy into the flashlampwas held below 100 joules to insure long life. The RLC circuit describedproduced a flashlamp pulse duration of about 800 seconds.

Although the operating characteristics of lasers are determined by theproperties of their active lasing ions, the actual results achieved inany given system is highly dependent on imperfections in the crystal.Microscopic imperfections invisible to the eye may make laseroscillation impractical. The presence of bubbles or inclusions mayscatter the beam and increase threshold significantly.

Despite the small rod size of the CaY Nd (SiO O crystal and its pooroptical quality, room temperature laser action was obtained at thethreshold of approximately 50 joules. With larger, high-qualitycrystals, threshold values of 5 joules should be possible.

Measurements of the laser threshold were accomplished by aligning thelaser rod in the laser head in the usual way with external reflectors. AIP25 phototube was then placed in the path of the beam with a 1.06micron interference filter between the laser and phototube to reduce thebackground signal. The phototube output was displayed on a Tektronixtype 555 dual-beam oscilloscope with one trace serving as an expandedscale. Thresholds could be accurately determined since the onset oflasing action appears as characteristic spikes as seen with othermaterials such as ruby.

Spectroscopic data on the fluorescence and absorption of CaY Nd (SiO Oare shown in FIGS. 3 and 4. The uncorrected fluorescence spectrum (FIG.3), from the above crystal shows in the near infrared including the 1.06micron emission corresponding to the Nd laser line. In CaY Nd (SiO Othis line is about ten times broader than in neodymium doped calciumfluorophosphate (50 A. vs. 6.5 A). Hence our new silicate oxyapatitehost, doped with neodymium should have enhanced energy-storagecapabilities making it very promising for Q-switching laserapplications. The visible and near infrared absorption spectrum (FIG. 4)for a single crystal of CaY Nd (SiO O indicates the considerable overlapof absorption bands to be found in this laser material. This suggests arelatively high average absorption of pump radiation. The relativeintensity of the 1.06 micron line in its two polarizations (I /1,) forthis sample was 1.4 and the decay time was 143 microseconds for the Ndemission.

The excitation spectrum (FIG. 5) of the 1.06 emission line shows thatenergy is transferred from the absorbing levels to the 4 state, theinitial laser level. 1

The melting point of neodymium doped CaY (SiO.;) O is significantlyhigher than neodymium doped Ca (PO )F. We have measured the hardness ofpolycrystalline samples of these two materials and compare them in thefollowing table:

Thermal shock resistance of neodymium doped CaY (SiO 30 is also high.All data indicate that CaY (SiO :Nd should be superior to doped calciumfluorophosphate in resisting structural distortion and failure at highpump levels.

Absorption spectra was measured on a Cary Model 14 commercialspectrometer. The excitation and fluorescence spectrometer systemconsisted of two grating monochromators for dispersing the excitinglight and the fluorescence light, along with associated optics,detectors, lamps and electronics. The source used was an Osram TypeXBO-900, a high pressure xenon arc lamp which was operated from a DCsupply having less than 1% ripple. Fluorescence measurements were madeusing a Jarrell- Ash monochromator. A 600' l/mm. grating blazed at 4000A. allowed excitation spectra to be taken from 2500 to 10,000 A. Thequantum detectors used RCA 7102 photomultipliers cooled to liquid Ntemperature.

We claim:

1. A composition of matter comprising a silicate oxyapatite host havingthe formula CaY (SiO O, said host containing an activator ion selectedfrom the group consisting of Nd and Er in the ion concentration range of.02 to 20 atom percent of the calcium and yttrium cations in the host.

2. The composition of claim 1 wherein the host contains the activatorion Nd in the ion concentration range of .02 to 6 atom percent.

3. A silicate oxyapatite laser crystal having the empirical formula CaY(SiO O:A ,S wherein A is the ion Er, S is the sensitizer ion Yb, x has avalue between 0.001 and 1 and y has a value between 0 and (4x).

4. The laser crystal of claim 3, wherein y has a value between 0 to land x has a value between 0.001 and0.30.

5. The laser crystal of claim 4 wherein y=0.

6. A silicate oxyapatite laser crystal having the empirical formula CaY(SiO O:A, S, where A is the ion Nd, S is the sensitizer ion Mn, whereinA is present in the ion concentration range of 0.02 to 20 atom percentof the calcium and yttrium cations in the formula and S is present inthe ion concentration range of 0 to 20 atom percent of the calcium andyttrium cations in the formula.

7. The laser crystal of claim 6 wherein the ion concentration of 8:0.

References Cited Ito: Silicate Apatites and Oxyapatites53 AmericanMineralogist pages 894 and 895 June 1968 Copy in Patent Ofiice SearchCenter.

5 ROBERT D. EDMO'NDS, Primary Examiner

